The present application relates to the control of air-handling units of a heating, ventilation and air conditioning system, and more particularly to regulating the amount of outdoor air that is introduced into the system in order to reduce the amount of mechanical heating and cooling required.
FIG. 2 conceptually illustrates a single duct air-handling unit (AHU) 10 of a heating, ventilation and air conditioning (HVAC) system which controls the environment of a room 12 in a building 5. Air from room 12 is drawn into a return duct 14 from which some of the air flows through a return damper 16 to a supply duct 18. Some of the return air may be exhausted outside the building through an outlet damper 20 and replenished by fresh outdoor air entering through an inlet damper 22. A minimum amount of fresh outdoor air entering the system for proper ventilation within the building is typically required by building codes. The dampers 16, 20, and 22 are opened and closed by actuators which are operated by a controller 24 to control the ratio of return air to fresh outdoor air. The mixture of return air and fresh outdoor air is forced by a fan 25 through a cooling coil 26 and a heating coil 28 before being fed into room 12.
Controller 24 also operates a pair of valves 27 and 29 that regulate the flow of chilled fluid through cooling coil 26 and the flow of heated fluid through heating coil 28, depending upon whether the circulating air needs to be cooled or heated. Cooing coil 26 and heating coil 28 provide “mechanical” heating and cooling of the air and are referred to herein as “mechanical temperature control elements.” The amount of cooling or heating energy that is required to be provided by mechanical temperature control elements is referred to herein as a “mechanical load” of the HVAC system.
Sensors 30 and 32, respectively, measure the temperature and humidity of the outdoor air and provide signals to controller 24. Another pair of sensors 34 and 36, respectively, measure the temperature and humidity of the air in return duct 14. Additional temperature sensors 38 and 39 are located in the outlet of supply duct 18 and in room 12.
Controller 24 executes a software program that implements an air side economizer function that uses outdoor air to reduce the mechanical cooling requirements for air-handling unit 10. There are three air side economizer control strategies that are in common use: temperature, enthalpy, and temperature and enthalpy. These strategies control transitions between two air circulation modes: minimum outdoor air with mechanical cooling and maximum outdoor air with mechanical cooling.
In temperature economizer control, an outdoor air temperature is compared to the return temperature or to a switch-over threshold temperature. If mechanical cooling is required and the outdoor air temperature is greater than the return air temperature or the switch-over threshold temperature, then a minimum amount of outdoor air required for ventilation (e.g. 20% of room supply air) enters air-handling unit 10. If mechanical cooling is required and the outdoor air temperature is less than the return temperature or a switch over threshold temperature, then a maximum amount of outdoor air (e.g. 100%) enters air-handling unit 10. In this case, outlet damper 20 and inlet damper 22 are opened fully while return damper 16 is closed.
With enthalpy economizer control, the outdoor air enthalpy is compared with the return air enthalpy. If mechanical cooling is required and the outdoor air enthalpy is greater than the return air enthalpy, then the minimum amount of outdoor air required for ventilation enters the air-handling unit. Alternatively, when mechanical cooling is required and the outdoor air enthalpy is less than the return air enthalpy, then the maximum amount of outdoor air enters airhandling unit 10.
With the combined temperature and economizer control strategy, when mechanical cooling is required and the outdoor temperature is greater than the return temperature or the outdoor enthalpy is greater than the return enthalpy, the minimum amount of outdoor air required for ventilation is used. If mechanical cooling is required and the outdoor temperature is less than the return air temperature and the outdoor enthalpy is less than the return enthalpy, then the maximum amount of outdoor air enters the air-handling unit. The parameters of either strategy that uses enthalpy have to be adjusted to take into account geographical environmental variations.
There are a number of different processes that can be used to regulate dampers 16, 20, and 22 to control flow rate of outdoor air, such as, direct airflow measurement method or energy and mass balance method.
The direct airflow measurement method requires sensors that measure airflow rate, which enables the fraction of outdoor air in the supply air to be controlled with a feedback controller. Krarti, “Experimental Analysis of Measurement and Control Techniques of Outdoor Air Intake Rates in VAV Systems,” ASHRAE Transactions, Volume 106, Part 2, 2000, describes several well-know methods for directly measuring the outdoor air fraction.
Alternatively, the fraction of outdoor air in the room supply air can be determined by performing energy and mass balances. Drees, “Ventilation Airflow Measurement for ASHRAE Standard 62-1989,” ASHRAE Journal, October, 1992; Hays et al., “Indoor Air Quality Solutions and Strategies,” Mc-Graw Hill, Inc., pages 200-201, 1995; and Krarti (supra), describe methods for determining the fraction of outdoor air in the supply air based on a concentration balance for carbon dioxide. The fraction of outdoor air in the supply air is determined from the expression:
      f    oa    =                    C        ra            -              C        sa                            C        ra            -              C        oa            
where Cra is the carbon dioxide concentration of the return air, Csa is the carbon dioxide concentration of the supply air, and Coa is the carbon dioxide concentration of the outdoor air.
Performing mass balances on the water vapor and air entering and leaving the room gives:
      f    oa    =                    ω        ra            -              ω        ma                            ω        ra            -              ω        oa            
where ωra is the humidity ratio of the return air, ωma is the humidity ratio of the mixed air, and ωoa is the humidity ratio of the outdoor air.
Performing an energy and mass balance on the air entering and leaving the room gives:
      f    oa    =                    h        ra            -              h        ma                            h        ra            -              h        oa            
where hra is the enthalpy of the return air, hma is the enthalpy of the mixed air, and hoa is the enthalpy of the outdoor air.
Assuming constant specific heats for the return air, mixed air, and outdoor air yields:
      f    oa    =                    T        ra            -              T        ma                            T        ra            -              T        oa            
An estimate of the fraction of outdoor air in the supply air can be determined from a model of the airflow in the air-handling unit, as described by Seem et al., in “A Damper Control System for Preventing Reverse Airflow Through The Exhaust Air Damper of Variable-Air-Volume Air-Handling Units,” International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, Volume 6, Number 2, pp. 135-148, April 2000, which reviews equations for modeling the airflow in air-handling unit 10. See also U.S. Pat. No. 5,791,408. The descriptions in both documents are incorporated herein by reference. The desired damper position can be determined based on the desired fraction of outdoor air and the airflow model, where the desired damper position can be determined.
One-dimensional optimization is applied to the fraction of outdoor air in the supply air to determine the optimal fraction which provides the minimal mechanical cooling load. Any of several well-known optimization techniques may be employed, such as the ones described by Richard P. Brent in “Algorithms for Minimization without Derivatives,” Prentice-Hall Inc., Englewood Cliffs, N.J., 1973, or Forsythe, Malcolm, and Moler in “Computer Methods for Mathematical Computations,” Prentice Hall, Englewood Cliffs, N.J., 1977. Alternatively, the “fminband” function contained in the Matlab software package available from The Mathworks, Inc., Natick Mass. 01760 U.S.A., may be used to find the optimal fraction of outdoor air.
These control strategies have assumed that the reference value or system optimal performance level was given. The reference value is typically determined by a sensor. The reference value or optimal operating conditions for a HVAC system is difficult to determine under various dynamic parameters. One problem with economizer control is the accuracy of the sensors. Humidity sensing elements can be inaccurate and unstable, which causes the economizer cycle to operate inefficiently. It would be advantageous to provide an alternative control system that minimized the need for sensors. Further, it would be advantageous to provide an alternative control strategy were the reference value is unknown. It would also be advantageous to provide a system that uses an extremum seeking controller to enhance system performance.